*3.1. The Influence of Al Content on Lattice Parameters, Phases and Microhardness*

A schematic diagram of different basic crystal structures as a function of the Al portion of the coating's metallic content in at.% is shown in Figure 3. It should be mentioned that in publications, the formula Cr1−*x*Al*x*N or Al1−*x*Cr*x*N is sometimes used for the same coating in dependence on the Al content. Nowadays, the most commonly used terminology in industrial applications is the short name (AlCr)N for Al-rich coatings. The authors suggest that coatings with a chemical composition in at.% of Cr > Al should be named (CrAl)N. If Al > Cr, then the coating should be named (AlCr)N. (CrAl)N coatings always have an fcc crystal structure. This structure type is also referred to as B1, a NaCl structure or c in certain publications, depending on the convention. With increasing Al content, a phase evolution to an hcp crystal structure takes place, which is also referred to as B4, a ZnS-type structure, wurtzite, w, h or hcp in publications. In the following, fcc and hcp will be used.

**Figure 3.** Schematic diagram showing the basic crystal structure of (CrAl)N and (AlCr)N coatings as a function of the Al content.

XRD investigations of rf-sputtered coatings have shown that at 57 at.% Al, a pure fcc structure was obtained, while at 75 at.% Al, the hcp structure was observed. The transition range between fcc and fcc + hcp was in the range of 57 at.% < Almax < 67 at.% [49]. With pulsed closed-field magnetron sputtering, the formation of hcp phases was observed at an Al content of 64 at.%, whereas fcc phases were detected at up to 60.9 at.%, giving a transition range of 60.9 at.% < Almax < 64 at.% [59]. A systematic experimental investigation of (CrAl)N and (AlCr)N coatings synthesised using the cathodic arc method showed that the crystal structure changed from a pure fcc structure to a mixed-phase structure of fcc and hcp at an Al content of about 60–70 at.% of the metal content in the cathodes [60], whereas an fcc structure was observed at up to 71 at.% in the coating by the authors of [61].

The reported different maximum Al contents for the X-ray-diffraction-measured pure fcc phase differ over a range of about 60–70 at.%, as shown in Table 1.


The deposition process itself (source properties, parameters) has an influence at the maximum Al concentration on whether a pure fcc phase is obtained using X-ray diffraction, as will be shown for arc evaporation in Section 3.4.

Taking measurement uncertainties into account, a well-accepted maximum critical value of the transition is about 70 at.% Al [61–66]. It should be mentioned that coatings with Al contents of 65–70 at.% might contain some traces of hcp phases, which may, however, be difficult to detect using XRD. For example, this effect was observed with SAED (Selected Area Electron Diffraction) measurement of (AlCr)N coatings deposited using arc evaporation from cathodes of Al70Cr30 on sapphire [62]. The critical value of the transition from fcc to the mixed fcc plus hcp structure also depends slightly on the deposition conditions, influenced, for example, by the evaporator magnetic field set-up analogously to AlTiN [9], but bias and deposition pressure also have an influence on the "fine" structure in the area of the transition [45].

Figure 4 shows experimental results for the hardness and lattice parameters of arcdeposited (CrAl)N and (AlCr)N coatings [60,61]. Both studies show a hardness increase of about 60% compared to CrN for fcc-structured coatings in the range of Al 60–70 at.%. The differences in the absolute values are likely related to different deposition techniques and hardness measurement conditions. Furthermore, in the region of the mixed-phase structure of fcc + hcp at high Al-content, the hardness is equivalent to CrN. The lattice parameters decrease from 0.415–0.416 nm (CrN)) to 0.413 nm in the region of Al 60–70 at.%.

**Figure 4.** Hardness and lattice parameters of (CrAl)N and (AlCr)N coatings versus Al content deposited by cathodic vacuum arc, redrawn after [60,61], original © Elsevier.

Lattice parameters for (CrAl)N, (AlCr)N, CrN and AlN coatings calculated ab initio are shown in Figure 5 and are compared with experimental XRD results [63]. The good fit of the calculated and the experimental data shows how well the basic properties of AlCr-based coatings can be calculated.
